Go back to basics when calculating the greenhouse impact and carbon neutrality of biofuels, researchers urge

Statements about biofuels being carbon neutral should be taken with a grain of salt. This is according to a study of researchers at the University of Michigan Energy Institute after completing a retrospective, national-scale evaluation of the environmental effect of substituting petroleum fuels with biofuels in the US. America’s biofuel use to date has in fact led to a net increase in carbon dioxide emissions, says lead author John DeCicco in Springer’s journal Climatic Change.

The use of liquid biofuels in the transport sector has expanded over the past decade in response to policies such as the US Renewable Fuel Standard (RFS) and California’s Low-Carbon Fuel Standard (LCFS). These policies are based on the belief that biofuels are inherently carbon neutral, meaning that only production-related greenhouse gas emissions need to be tallied when comparing them to fossil fuels.

This assumption is embedded in the lifecycle analysis modelling approach used to justify and administer such policies. Simply put, because plants absorb carbon dioxide as they grow, crops grown for biofuels should absorb the carbon dioxide that comes from burning the fuels they produce. Using this approach, it is often found that crop-based biofuels such as corn ethanol and biodiesel offer at least modest net greenhouse gas reductions relative to petroleum fuels.

Field data for assessing the net carbon dioxide emission effect of biofuels has been available since the Renewable Fuel Standard was passed in 2005. DeCicco’s team evaluated the data up to 2013, using the Annual Basis Carbon (ABC) accounting method he previously developed. It takes a circumscribed look at the changes in carbon flows directly associated with a vehicle-fuel system, and does not treat biofuels as inherently carbon neutral.

Instead, the ABC method tallies carbon dioxide emissions on the basis of chemistry in the specific locations where they occur. The system takes into account motor fuel consumption, fuel processing operations and resource inputs, including the use of cropland for biofuel feedstocks. Unlike lifecycle analysis, ABC accounting reflects the stock-and-flow nature of the carbon cycle, recognizing that changes in the atmospheric stock depend on both inflows and outflows.

DeCicco’s team found that the gains in carbon dioxide uptake by feedstock, such as corn, were enough to offset biofuel-related biogenic emissions by only 37 percent, rather than 100 percent, during the period 2005 to 2013. “This shows that biofuel use fell well short of being carbon neutral even before considering process emissions,” says DeCicco. In this regard, the researchers concluded that rising US biofuel use has led to a net increase rather than a net decrease in CO2 emissions. This finding contrasts with those of lifecycle analysis models which indicate that crop-based biofuels such as corn ethanol and soy biodiesel lead to a modest reduction in greenhouse gas emissions. DeCicco’s work demonstrates that it is possible to empirically evaluate the necessary condition for a biofuel to offer carbon dioxide mitigation benefits. “Doing so provides a bounding result that suggests a need for much greater caution regarding the role of biofuels in climate mitigation,” DeCicco concludes.

A new study from University of Michigan researchers challenges the widely held assumption that biofuels such as ethanol and biodiesel are inherently carbon neutral. Contrary to popular belief, the heat-trapping carbon dioxide gas emitted when biofuels are burned is not fully balanced by the CO2 uptake that occurs as the plants grow, according to a study by research professor John DeCicco and co-authors at the U-M Energy Institute.

The study, based on U.S. Department of Agriculture crop-production data, shows that during the period when U.S. biofuel production rapidly ramped up, the increased carbon dioxide uptake by the crops was only enough to offset 37 percent of the CO2 emissions due to biofuel combustion. The researchers conclude that rising biofuel use has been associated with a net increase—rather than a net decrease, as many have claimed—in the carbon dioxide emissions that cause global warming. The findings were published online Aug. 25 in the journal Climatic Change. „This is the first study to carefully examine the carbon on farmland when biofuels are grown, instead of just making assumptions about it,“ DeCicco said. „When you look at what’s actually happening on the land, you find that not enough carbon is being removed from the atmosphere to balance what’s coming out of the tailpipe.“

The use of biofuels to displace petroleum has expanded over the last decade in response to policies, such as the U.S. Renewable Fuel Standard, that promote their use for transportation. Consumption of liquid biofuels—mainly corn ethanol and biodiesel—has grown in the United States from 4.2 billion gallons in 2005 to 14.6 billion gallons in 2013. The environmental justification rests on the assumption that biofuels, as renewable alternatives to fossil fuels, are inherently carbon neutral because the carbon dioxide released when they are burned was derived from CO2 that the growing corn or soybean plants pulled from the atmosphere through photosynthesis. That assumption is embedded in the carbon footprint models used to justify and administer policies such as the federal RFS and the California Low-Carbon Fuel Standard. The models, which are based on a technique called lifecycle analysis, have often found that crop-based biofuels offer at least modest net greenhouse gas reductions relative to petroleum fuels.

Instead of modeling the emissions, DeCicco and his colleagues analyzed real-world data on crop production, biofuel production, fossil fuel production and vehicle emissions—without presuming that that biofuels are carbon neutral. Their empirical work reached a striking conclusion. „When it comes to the emissions that cause global warming, it turns out that biofuels are worse than gasoline,“ DeCicco said. „So the underpinnings of policies used to promote biofuels for reasons of climate have now been proven to be scientifically incorrect. „Policymakers should reconsider their support for biofuels. This issue has been debated for many years. What’s new here is that hard data, straight from America’s croplands, now confirm the worst fears about the harm that biofuels do to the planet.“

The Climatic Change paper is titled „Carbon balance effects of U.S. biofuel production and use.“ DeCicco’s co-authors include current and former students at the U-M School of Natural Resources and Environment and the U-M Program in the Environment, as well as a postdoctoral researcher at the Energy Institute. They are Danielle Yuqiao Liu, Joonghyeok Heo, Rashmi Krishnan, Angelika Kurthen and Louise Wang. Some funding for the study was provided by the American Petroleum Institute.